1. Field of the Invention
The present invention relates to an optical fiber amplifier for a light transmission system using an optical fiber as a transmission route, and more particularly, relates to an optical fiber amplifier capable of amplifying optical signals so that the gains of the signals are made equal to each other, and an optical fiber transmission system employing such an optical fiber amplifier.
2. Description of the Related Art
A conventional multi-wavelength optical fiber amplifier or bidirectional optical fiber amplifier includes, at least, a rare earth doped optical fiber, a pump laser diode emitting pump light for exciting the rare earth ions, and an optical coupler for coupling the pump light to the rare earth doped optical fiber. When a signal is input into the optical fiber amplifier with the above configuration, the signal is amplified due to stimulated emission caused by the rare earth ions of a population inverted state. When a plurality of signals are attempted to be amplified simultaneously by a same rare earth doped optical fiber, the gains of the signals vary depending on the wavelengths thereof because the absorption and emission cross sections of the rare earth ions vary depending on the wavelengths.
Methods for suppressing such a variation in the gains depending on the wavelength have been reported. For example, B. Pedersen et al., "The Design of Erbium-Doped Fiber Amplifiers", Journal of Lightwave Technology, Vol.9, No.9, pp.1105-1112 (1991) describes that the wavelength dependency of the absorption and emission cross sections of a rare earth doped optical fiber, which is the cause of the gain variation, can be reduced by using aluminum ions as a codopant.
Using an acousto-optical element to suppress the gain variation is proposed in OFC/IOOC '93, Paper ThD2, 1993, and using a fiber grating is proposed in OFC '91, Paper PD20-1, 1991. E. L. Goldstein et al. "Suppression of Dynamic Cross-saturation in Multiwavelength Lightwave Networks with Inhomogeneously Broadened Fiber Amplifiers", OAA '93, Paper SuE3-1, pp.70-73, 1993 proposes to connect a plurality of optical fiber amplifiers in series so that the gain and wavelength characteristics can be cancelled each other.
In any of the above proposals, however, the gains of signals are made equal only when the input powers of the signals are equal. The idea of equalizing the gains of signals with different input powers has not been examined.
The input power of an analog signal input into an optical fiber amplifier is large, while that of a digital signal is small. Accordingly, when the analog signal and the digital signal are to be amplified simultaneously as in the case of a bidirectional optical amplifier, this large power difference basically causes the gains of the respective signals to vary from each other. Further, in the case where a plurality of optical fiber amplifiers are connected in series to be used for a distribution system, the gain variation causes a lack of power at light receiving section because each signal receives an equal distribution loss. As a result, transmission characteristics of the signals degrade.